The present invention relates to a method of and apparatus for moulding an assembly of a thermoplastics member with a fusion pad, the latter being intended for the purpose of welding the assembly to a surface of compatible thermoplastics material. More particularly, the invention concerns a thermoplastics member in which the fusion pad comprises an element which is intended to be electrically heated to effect the aforementioned welding. Typical examples of assemblies having thermoplastics members with fusion pads are to be found for the jointing or tapping of thermoplastics piping; in a pipe joint or coupling the thermoplastics member may comprise a socket within which a tubular fusion pad is disposed to effect the welding between the socket and a pipe received therein. For a pipe tapping, the thermoplastics member may be in the form of a tapping saddle for connecting two pipes together at an angle to one another with one pipe communicating through the wall of the other pipe. A tapping saddle is particularly useful for connecting together pipes intended to carry a liquid or gas such as for example pipes forming part of a gas or water main system. FIGS. 1 and 2 of the accompanying illustrative drawings show a known form of tapping saddle in which FIG. 1 is a perspective view from below of the tapping saddle, and FIG. 2 is a perspective view from above of the tapping saddle connected to a pipe. In those Figures the tapping saddle has a saddle pad 2 of thermoplastics for connection to a main pipe 4, a tubular body 6 leading from the pad 2, and a branch pipe 7 leading from the body 6 for connection to a pipe 8. In the case of a gas or water main, the pipe 4 is the main pipe and the pipe 8 leads to a consumer point such as for example a house.
It will be appreciated that the pad 2 and the corresponding region of the wall of the pipe 4 have to be heated in order that they may be fused together; for this purpose a fusion pad part 2a having an electrical heating coil is located in the saddle pad 2. This can be achieved by winding a plastics coated wire into a flat spiral, heat sealing this spiral into a single piece that can be curved to the required pipe diameter, and fixing terminals to the two ends of the heat sealed spiral. This spiral, complete with the terminals, is curved to the required pipe diameter before spiral and terminals are loaded into an injection moulding tool for encapsulation in thermoplastics to form the saddle pad 2 and body 6. The plastics coating which encapsulates the wire spiral is used to maintain the correct wire spacing between adjacent ends of the spiral, and also the optimum spacing between the coil and the pipe 4 during welding.
Our G.B. Patent Specification 2,158,007A discloses a method of producing a fusion pad comprising forming in one surface of a thermoplastics sheet a continuous groove extending between an inner end and an outer end, locating in the groove and electrically conductive wire having a diameter less than the depth of the groove, and melting the upper portion of the groove to envelope and retain the wire in the sheet. The two ends of the wire are connected to respective terminals which project from the sheet. This fusion pad is then bent into a required curvature (such as a saddle) and is placed in a suitable tool to mould a tapping saddle which includes the fusion pad. Experience has indicated that although the fusion pad is attached to the tapping saddle assembly, there is not always a completely homogeneous interface and sometimes it may be possible to prise the fusion pad from the tapping saddle assembly.
In use the known tapping saddle (including the fusion pad) is fused to the pipe 4 by passing current through the wire in the pad. In order to make this connection it has been found necessary to pass a comparatively long burst of current through the wire. This is because the wire has to heat both the pipe and also the part of the pad behind the wire in order to complete the bond of the pad to the tapping saddle. It has been found that this comparatively long duration current can produce sufficient heat to cause the edges of the saddle pad to lift off the pipe 4 thereby reducing the efficiency of the bond.
It is an aim of the invention to alleviate the aforementioned disadvantages.